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    <script>

        // function code(n) {
        //     // 短路赋值
        //     n = n || 4 ;
        // }

        // code() ;


        // function sum(a , b) {
        //     a = a || 0 ;
        //     b = b || 0 ;
        //     return a + b ;
        // }


        function sum(a = 0 , b = 0) {
            return a + b ;
        }

        console.log(sum());   //

    </script>
    
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